High molecular weight Dextran will be used to produce the hyperviscosity syndrome in monkeys. This will produce the situation as occuring in such diseases as multiple myeloma and diabetes, resulting in a retinopathy with the formation of retinal capillary microaneurysms. These microaneurysms will be studied both by fluoresein angiography and electron microscopy. The correlation of these two parameters will allow us to study the exact stages of the development of retinal capillary microaneurysms.